Objective: The goal of this study is to establish the mechanism(s) of progression of primarily spinal syringomyelia (PSS), the type of syringomyelia that is associated with pathology in the spinal column and not at the craniocervical junction. Our hypothesis is that when a lesion obstructs the spinal subarachnoid space, it shortens the segment of spinal canal that dampens the CSF pressure waves that are produced with each heartbeat and creates enlarged spinal subarachnoid pressure waves that act on the spinal cord above the block to drive CSF into the spinal cord and create a syrinx. After a syrinx is formed, enlarged subarachnoid pressure waves compress the external surface of the spinal cord, propel the syrinx fluid, and promote syrinx progression.